Closed loop ferroresonant regulator

ABSTRACT

The transformer core of a ferroresonant regulator includes a main core portion on which is wound the output winding, a control portion on which is wound a compound winding, and a saturating portion. The magnetic flux in the main core portion divides, part into the control portion and part into the saturating portion. The output voltage is controlled without saturation of the main core portion by a low current in the compound winding. An integrating circuit including an integrating capacitor is coupled to the output winding to develop a voltage proportional to the volt-time integral of the output voltage. A switch responsive to the integrating capacitor voltage causes current to flow in the compound winding. The current through the compound winding opposes saturation of the control core portion and precipitates saturation of the saturating core portion to limit the total flux swing in the main core portion and the half cyclic average of output voltage. The operation of the switch in conjunction with the saturation of the saturating core portion causes the ferrocapacitor to reverse its charge each half cycle, and ferroresonant regulation is maintained. The output voltage may be varied by varying the rate of charge of the integrating capacitor, and closed loop regulation may be achieved by adding a feedback network responsive to the load voltage for varying the charging rate.

United States Patent 2,734,164 2/ 1956 Knowlton 3,079,546 2/1963 Kuba3,183,429 5/1965 Baycura et al.

lnventors Appl. No.

Filed Patented Assignee CLOSED LOOP FERRORESONANT REGULATOR 14 Claims, 6Drawing Figs.

US. Cl 323/56, 321/25, 323/60 Int. Cl t. G05f 1/46, G05f l/64 Field ofSearch 323/48, 50,

References Cited UNITED STATES PATENTS Primary Examiner-J. D. MillerAssistant Examiner-A. D. Pellinen AttorneysR. J. Guenther and E. W.Adams, Jr.

ABSTRACT: The transformer core of a ferroresonant regulator includes amain core portion on which is wound the output winding, a controlportion on which is wound a compound winding, and a saturating portion.The magnetic flux in the main core portion divides, part into thecontrol portion and part into the saturating portion. The output voltageis controlled without saturation of the main core portion by a lowcurrent in the compound winding. An integrating circuit including anintegrating capacitor is coupled to the output winding to develop avoltage proportional to the volt-time integral of the output voltage. Aswitch responsive to the integrating capacitor voltage causes current toflow in the compound winding. The current through the compound windingopposes saturation of the control core portion and precipitatessaturation of the saturating core portion to limit the total flux swingin the main core portion and the half cyclic average of output voltage.The operation of the switch in conjunction with the saturation of thesaturating core portion causes the ferrocapacitor to reverse its chargeeach half cycle, and ferroresonant regulation is maintained. The outputvoltage may be varied by varying the rate of charge of the integratingcapacitor, and closed loop regulation may be achieved by adding afeedback network responsive to the load voltage for varying the chargingrate.

' 4 sheets -sheet z v FIG. 2 He. 3

Patented April 6, 1971 I 3,573,605

4 Sheets-Sheet 5 AAA FIG. 5

Patented April 6, 1971 3,573,605

4 Sheets-Sheet 4 I V U I I FIG. 6

1 CLOSED LOOP FERRORESONANT REGULATOR CROSS-REFERENCE TO RELATEDAPPLICATION This application is a continuation-in-part of our copending5 application Ser. No. 763,882, filed Sept. 30, 1968, now abandoned.

BACKGROUND OF THE INVENTION LII regulator and the saturating inductorshunts the output. The 1 capacitor, often called a ferroresonatingcapacitor, or more simply a ferrocapacitor, shunts the saturatinginductor and is usually tuned near resonance with the linear inductance.Alternatively, the two inductors may be wound upon a single transformercore and the input and output electrically isolated. In that case, theinput winding is on a nonsaturating portion of the transformer coreand'the output winding is on a saturating portion. With eitherconstruction, in each half cycle of AC input the saturating coresaturates, and the impedance of the saturating winding drops. Thecapacitor resonates with the low saturated inductance to quicklydischarge through the saturating winding and recharge in the oppositepolarity. The core thereupon drops out of saturation so that furtherringing does not occur. The AC output, which may be rectified to provideDC output, is taken from across the ferrocapacitor. When theferrocapacitor voltage reverses, therefore, the output voltage reverses,and the output half cycle is terminated. A saturating core, however,requires a'fixed volt-time area of its saturating winding characteristicin order to saturate. Consequently, when the input voltage increases ordecreases, the core saturates earlier or later in the immediate halfcycle, but the volt-time product of each half cycle of output voltage isconstant. When the input frequency is constant, therefore, providing aconstant steady state and average time period per output half cycle, theoutput voltage must be constant. As a result, changes in input voltagehave little effect on output voltage and regulation against changes ininput voltage is obtained thereby.

The advantages of these prior art circuits are well known. They may bemade very efficient, simple and reliable; they provide good outputvoltage regulation with changes in line voltage, input noisesuppression, inherent output short circuit protection, good input powerfactor, and a relatively square output waveform which is particularlywell suited for rectifying and filtering.

These ferroresonant circuits are, however, subject to severaldisadvantages. The idealized expression for average induced outputvoltage is generally given as E -4ANFB X 10"where A is the cross sectionarea of the saturating core, N is the number of turns in the outputwinding, F is the frequency, and B is the flux density required tosaturate the core. As can be seen from the foregoing equation, theoutput voltage of a ferroresonant regulator is particularly sensitive toinput supply frequency changes. In addition, since the equationrepresents induced output voltage, voltage drops in the output circuitdue to output current are not compensated for, and output terminalvoltage is not regulated for changes in load. Furthermore, since theoutput voltage depends upon specific core properties and dimensions, thecore manufacturing tolerances directly affect output voltage tolerances.Finally, ferroresonant transformers generate high external magneticfields because of the saturated cores, particularly at light loads whenthe core is driven deeper into saturation.

It has not heretofore proved a simple task to add output voltageregulation with load and frequency to a ferroresonant circuit withoutdestroying or duplicating the ferroresonant function because thephysical characteristics of the saturable lien core itself largelydetermine the regulation. Approaches which short-out a winding on themain core at a variable time in the input cycle to attain regulationdestroy ferroresonant action by preventing core saturation anddischarging the ferrocapacitor. The ferroresonant regulationdeteriorates into pulse width modulated switching regulation, whichgives an output wave inherently difficult to filter when rectified.Approaches which add a variable impedance series regulator in serieswith a ferroresonant regulator wastefully duplicate the ferroresonantfunction of output regulation with input variations.

An object of this invention is, therefore, to add efiiciently outputvoltage regulation with load and frequency variations to the basicferroresonant regulating action.

Another object is to integrate closed loop feedback into a ferroresonantcircuit.

Still another object is to provide ferroresonant type voltage regulationwithout the usual high level magnetic field surrounding the transformer.

4 SUMMARY OF THE INVENTION In the present invention, variable outputvoltage and closed loop regulation are added to a transfonnerferroresonant voltage regulator by precipitating saturation of a smallcore portion, rather than the entire transformer core, to determine thetime in the cycle when the charge on the ferrocapacitor is reversed. Thesmall core portion may comprise a single outer leg or a pair ofsaturating shunts. An integrating circuit includ ing an integratingcapacitor is coupled to the secondary winding to develop a voltageproportional to the volt-time integral of the voltage across theferrocapacitor. A switch operates in response to a predetermined voltageacross the integrating capacitor to cause current to flow in a low powercompound winding. The switch may connect the compound winding in serieswith a saturating winding to a source of voltage in phase with theferrocapacitor voltage, or it may cause induced current to flow bymerely shorting the compound winding. The resulting compound windingcurrent generates core flux that opposes the existing flux linking thecompound winding to force the small core portion to saturate Once thecore portion has saturated, the main flux which links the output windingis restricted, causing termination of the half cycle and reversal of thecharge on the ferrocapacitor. Ferroresonant regulation is therebyprovided with only a small portion of the transformer core saturatingand only a small amount of current through the switch. Both theelectronic switch and the transformer are rthereby made more efficientand less expensive. With this novel structure, saturation of the coreportion and termination of the half cycle is thus precipitated to setthe volt-time integral of each half cycle of output waveform when thecharge on the integrating capacitor reaches the predetermined value.Therefore, output voltage may be varied by varying the charging rate ofthe integrating capacitor. Closed loop regulation may then be added by afeedback network responsive to the load voltage for varying the chargingrate of the integrating capacitor. A high degree of regulation ofvoltage with changing input voltage and frequency and changing load isthereby obtained very simply, inexpensively and efiiciently.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of anembodiment of the invention;

FIGS. 2 and 3 are plots of various voltages against a common timeabscissa which are helpful in explaining the operalOl035 025 DETAILEDDESCRIPTION In the circuit of FIG. 1, a transformer 11 has a center leg12, and two outer legs 13 and 14. On center leg 12 there is wound aprimary winding 16, a secondary winding 17 and a third winding 18.Magnetic shunts l9 and 20 including airgaps 21 separate the primarywinding from the other two windings to provide a path for leakage flux,and thereby to reduce the primary-secondary coupling in the mannercommon in ferroresonant circuits.

The AC terminals of a full-wave bridge rectifier 22 are connected acrosswinding 18, and a pair of DC output terminals 23 and 24 are connected tothe DC terminals of bridge 22. A filter capacitor 25 is connected acrossthe output terminals. A ferrocapacitor 26 is connected across secondarywinding 17; a pair of compound windings 27 and 28 on outer legs 13 and14, respectively, are connected in series-aiding relationship in se'ries with triac 29, across a portion of secondary winding 17.

Triac 29 is a three-terminal bilateral triode switch which is capable ofpassing current in either direction in response to the application of arelatively low-current low-voltage pulse between its gate and cathodeterminals. Such a switch is described in detail at pages 142 throughI48, 245 and 279 of the texVSemiconductor Controlled Rectifiers:Principles and Applications of P-N-P-N Devices" by F. E. Gentry et al.,copyright 1964. Obviously, the invention is not limited to the use ofsuch devices, however, as any equivalent device or combination ofdevices could be substituted therefor.

An integrating circuit comprising the series combination of a resistor31 and an integrating capacitor 32 is connected across another portionof winding 17 A pair of Zener diodes 33 and 34, connected in series andpoled in opposite directions, connects the junction of capacitor 32 andresistor 31 to the gate electrode of triac 29. The AC terminals of afullwave bridge rectifier 36 are connected across resistor 31. The DCtenninals of bridge 36 are connected across the emittercollector path ofa transistor 37. Potentiometer 38 is connected across output terminals23 and 24, its tap being connected to the base of transistor 37.Finally, a Zener diode 39 is connected between output terminal 24 andthe emitter electrode of transistor 37.

If triac 29 is disabled so that compound windings 27 and 28 are heldopen, the circuit performs in a typical ferroresonant manner. In eachhalf cycle the entire 'portion of the transformer core below magneticshunts 19 and 20 saturates at a fixed volt-time integral of voltage onwinding 17. Ferrocapacitor 26 resonates with the low saturatedinductance of winding 17, and discharges and recharges in the oppositepolarity, the capacitor current flowing through winding 17 Furtherringing is prevented because the core drops out of saturation and thehigher inductance is restored to winding 17 Ifthe voltage on primarywinding 16 increases, the core saturates earlier to terminate theimmediate half cycle, and the average output volt-v age remains constantas discussed before. The magnetic shunt 19 provides a path for excessprimary flux which does not link the secondary and third windings toallow their voltages to remain constant while the input voltage varies.Since output winding 18 is closely coupled to winding 17, theirrespective voltages remain in phase and in proportion. The DC outputvoltage at terminal 23 and 24 is therefore regulated with changes ininput voltage.

The operation of the compound windings, the triac and the integratingcircuit may be better understood with reference to the curves of FIG. 2;waveform A is the voltage across output winding 18; waveform B is thevoltage across integrating capacitor 32; waveform C is the voltageacross triac 29; waveform D is the voltage across compound winding 28;and wavefonn E is the current through triac 29. All of the waveforms areplotted against a common time abscissa.

Since the integrating circuit which comprises resistor 31 and capacitor32 is connected across a portion of winding 17, the voltage acrossintegrating capacitor 32 is proportional to the volt-time integral ofthe voltage across winding 18. As can be seen in FIG. 2, waveform B,this is an alternating voltage of trapezoidal waveform. Intersectingwaveform B are shownthe reverse breakdown potentials, 41 and 42, of theZener diodes 33 and 34, respectively. When the integrating capacitorvoltage reaches the breakdown potential of the Zener diode that iscurrently back-biased, triac 29 fires to connect compound windings 27and 28 across a portion of winding 17. This occurs at time t, in FIG. 2,while the flux in the transformer core is uniformly increasing. Anysource of AC voltage in phase with that on winding 17 may be used, butvoltage from taps on winding 17 are convenient. As can be seen fromwaveforms C and D, when the triac fires, its potential drops to almostzero and the voltage across compound winding 28 jumps to its maximumvalue. Because windings 27 and 28 are series aiding, the triac currentproduces an increment of flux which is directed down in one winding andup in the other. These directions are illustrated in FIG. I by arrows 50and 51, respectively. The main magnetic flux in the center leg 12 splitsinto legs 13 and 14, as shown by arrows 52 and 53, respectively. Theflux due to compound winding 28 therefore, aids the main flux, and thatdue to compound winding 27 opposes the main flux. As a consequence, whentriac 29 is fired, the saturation of leg 14 is speeded up and that ofleg 13 is prevented. The decrease of flux in center leg 12 due towinding 27, however, is balanced by the increase due to winding 28, andthe voltage across winding 17 remains unaffected. The flux in leg 14continues to increase to saturation at time 1 after which it cannotincrease. At this point, the impedance of winding 28 drops, and theincrement of flux due to the triac current in winding 27 increases.Since that increment is no longer opposed by an increment from winding28, it causes the heretofore steadily increasing flux in center leg 12to stop increasing, and the impedance of winding 17 to drop.Ferrocapacitor 26 thereupon discharges primarily through winding 17, andrecharges in opposite polarity in typical ferroresonant fashion. Thedischarge of ferrocapacitor 26 starts at time t and the rechargecontinues until time 1 Triac 29 continues to conduct through windings 27and 28 until it is able to turn itself off at time Its voltage thenimmediately rises to reflect the voltage across winding 17. i

The time in the cycle when core 14 saturates, therefore, determines thetime in the cycle when ferrocapacitor 26 reverses its charge andtherefore determines the output voltage, just as the core dimensions ofa typical ferroresonant transformer determines the time itsferrocapacitor reverses its charge and determines its output voltage. Ifthe reversal occurs earlier in the cycle, output voltage is reduced, andvice versa. Since transformer leg 14 requires a fixed volt-time area tosaturate if triac 29 fires at a fixed volt-time integral, a constantvoltage is maintained at the output of winding 18 with changing inputconditions. The circuit of the invention therefore perfonns with all theadvantages of a typical ferroresonant regulator, but only a portion ofthe transformer core saturates. As a consequence, core losses areconsiderably less and the external magnetic field is greatly reduced;this provides much improved efficiency at light loads.

A major advantage of providing adjustable output voltage may be obtainedin addition to the foregoing advantages if the integrating resistance ismade variable. The waveforms of FIG. 3, which represent voltages andcurrents observed at the same circuit points of the correspondingwaveforms of FIG. 2, illustrate the performance when integratingresistor 31 is reduced in value. It will be noted that the triac firesearlier in the cycle at time Time 2 has not changed position becausefrequency must remain that of the driving source applied to primarywinding 16, and the saturation of core 28 determines the end of eachhalf cycle. The shaded areas under the waveforms D in FIGS. 2 and 3represent the volt-time integral of winding 28. Since a fixed volt-timeintegral is required to saturate core leg 14, the shaded area undercurve in FIG. 3 must equal that under curve in FIG. 2. With a smallercutout of the shaded area in FIG. 3 due to voltage across the triac, thetwo areas can only be the same if the amplitude of the waveform D ofFIG. 3 is lower. Thus a lower value of integrating resistance provides alower output voltage.

The purpose of bridge rectifier 36, transistor 37, Zener diode 39 andpotentiometer 38 is to vary the effective integrating resistance as afunction of output voltage and thereby to provide closed loop feedbackregulation. The AC voltage appearing across resistor 31 is rectified bybridge 36 and appears across the collector-emitter path of transistor37.

Zener diode 39 holds the emitter to a constant reference voltage.Changes in output voltage appearing across terminals 23 and 24 appearalso, in proportion according to the setting of the tap of potentiometer38, on the base of transistor 37 to vary the transistors bias. As thebias is thus varied, because of a change in output terminal voltage, theconductivity of the collector-emitter path which shunts resistor 31 isvaried, and therefore the integrating resistance.

The feedback circuit operates to compensate for changes in load andfrequency as follows: when the terminal output voltage tends to increasebecause of a decrease in load or increase in frequency or input voltage,the positive bias on transistor 37 is increased to make the transistormore conductive. With a more conductive shunt across resistor 31, thetotal integrating resistance is reduced, and integrating capacitor 32charges more quickly. As a consequence, triac 29 is fired earlier in thehalf cycle. As discussed heretofore, as the triac is fired earlier inthe half cycle, less voltage is imparted to the output in the halfcycle, and the output voltage tends to drop again.

Thus, closed loop feedback has been added to ferroresonant typeoperation to provide very close regulation of output voltage withchanging load as well as line conditions. What is perhaps moreimportant, however, is the simplicity and the efficiency with which itis accomplished according to the teachings of this invention. It wasnoted before that only a portion of the core is driven into saturationto control the entire flux. It should be further noted that the currentin the compound windings does not control the total flux, but merely anincrement which, when added to the main flux, is sufficient to controlthe saturation time. In addition, when the charge on the ferrocapacitoris reversed as a result of the saturation of core leg 14, the capacitorcurrent flows through winding 17 rather than through the compoundwindings and triac 29. As a consequence, a small triac current may beused to control a large output current. In a regulator constructed asherein described from which the output is 50 volts regulated to :1percent from 0 to 100 amperes, the triac current is less than amperes,and the full load efficiency approximately 90 percent. The regulator istruly low cost, simple and efficient.

The numbers of turns in the portion of winding 17 that is connectedacross the integrating circuit and the portion that is switched acrossthe compound windings are not critical. Each is chosen with properregard for the ratings of the component and the voltages required tofire the triac and to reverse the ferrocapacitor charge respectively.

The circuit of FIG. 4, which may be substituted for that part of FIG. 1that lies within the dotted rectangle 61, is an altemative arrangementthat eliminates one compound winding without jeopardizing theperformance of the regulator. In this case, triac 29 is connecteddirectly across compound winding 27 and serves to short the compoundwinding instead of connecting it to a voltage source. Transformer leg 14still saturates each half cycle, however, and the waveforms of FIGS. 2and 3 still apply. Just before time t the voltage across winding 18 issteady and the magnetic flux in all three transformer legs is steadilyincreasing. At time t,, the voltage across integrating capacitor 32having exceeded the breakdown voltage of the Zener diode pair 33, 34,triac 29 tires to short circuit winding 27. The short circuit causes acirculating current that virtually prevents the flux in leg 13 fromcontinuing to increase. The flux in center leg 12, however, continues toincrease at its previous rate, and the voltage across winding 18 remainssteady. The flux in leg 13 being clamped, the rate of increase of fluxin leg 14 is doubled in order to support the continued increase incenter leg flux. As a consequence, leg 14 saturates at time 1,, furtherincrease in the flux in center leg 12 is prevented, and theferrocapacitor reverses its charge through winding 17 as in theembodiment of FIG. 1.

The DC path through transistor 37 and bridge 36 requires isolationbetween the error detecting potentiometer 38 and the integratingresistor 31. Consequently, both may not be connected across the samewinding. Ferrocapacitor 26, however, may be connected across eitherwinding 17 or 18, or even a third winding closely coupled thereto.

Since the function of Zener diode 39 is to provide a reference voltagein the bias circuit of transistor 37, it may be placed in series withthe base of the transistor. The main consideration is the amount ofcurrent through the Zener diode needed to sustain breakdown. Inaddition, a resistor may be connected between the anode of diode 39 andterminal 23 for thermal stability.

A particularly advantageous embodiment of the invention that utilizesonly one compound winding is illustrated in FIG. 6. The circuit of FIG.6 also may be substituted for that part of FIG. I that lies within thedotted rectangle 61. Here compound winding 27 has been wound on thecenter leg rather than on an outer leg as in the circuits of FIGS. 1, 4and 5. Extending from center leg 12 to outer legs 13 and 14 betweencompound winding 27 and output winding 18 are a pair of laminatedsaturating magnetic shunts 63 and 64, respectively. Just before triac 29fires, the voltage on winding 18 is steady, and the magnetic flux in thetransformer legs is linearly increasing. When triac 29 fires, it shortcircuits compound winding 27 to prevent any further change in the fluxin the portion of center leg 12 that extends through winding 27. Anycontinued increase in flux in leg 12 through windings 17 and 18 isforced to pass through shunts 63 and 64. These shunts quickly saturate,however, to prevent any further increase in flux in center leg 12, andthe ferrocapacitor reverses its charge through winding 17 as in theprevious embodiments.

The embodiment of FIG. 6 has two major advantages over that of FIG. 4.The first is a saving of space. In the embodiment of FIG. 4, because thecompound winding 27 is wound on an outer leg, there must be spaceallotted between outer leg 13 and center leg 12 to accommodate thiswinding. Since the embodiments of FIG. 4 and FIG. 6 eliminate winding 28of FIG. I, however, there is no winding in the corresponding position onleg 14. As a result, if the core of the transformer is made symmetricalthere will be unused space between legs 12 and 14 opposite winding 27.In the alternative, if the transformer core is not symmetrical, therewill be unused space between legs 12 and 13 above winding 27. Theembodiment of FIG. 6 allows the space between outer and center core legsto be just enough to accommodate the windings on the center leg with noopen wasted space. The transformer may therefore be made more compact.Since the transformer occupies most of the space of the entire regulatorcircuit, the overall space saving is significant.

The second advantage of the embodiment of FIG. 6 lies in the techniquesof construction. A small airgap can greatly increase the drivingmagnetomotive force required to saturate a core section. To prevent anyunwanted airgap a core such as that of FIG. 4 is normally assembled inlayers each containing an E- and an I-shaped lamination with the jointbetween them staggered in successive layers. Such interleaving oflaminations is expensive. With a core such as that of FIG. 6, however,all of the windings are on one leg and a minute airgap is not critical.The core can therefore be constructed of stacks of laminations withoutinterleaving, resulting in considerable cost savings.

Finally, a further advantage of the embodiment of FIG. 6 is that thetotal volume of saturating shunts 63 and 64 is considerably less thanthat of core leg 14. With less saturating iron the circuit iselectrically more efiicient.

It will also be recognized by those skilled in the transformer art thatthe embodiment of FIGS. 4 and 6 may be constructed in the two-core formshown in FIG. 5. In place of primary winding 16 of FIG. 1, there is alinear inductor 116 in series with the AC source and winding 17. As inFIG. 1, the ferrocapacitor 26 is connected across winding 17. Theremaining components bear the same numbers and are connected in the samemanner as shown in FIGS. 1 and 4.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

We claim:

1. Voltage regulating apparatus comprising a transformer having amagnetic core, primary, secondary, and compound windings wound on saidcore, said core providing a path for magnetic flux linking said primary,secondary, and compound windings, nonsaturating flux shunting meansdisposed between said primary and secondary windings to shunt a portionof said magnetic flux from said primary winding, and saturating fluxshunting means disposed between said secondary and compound windings forproviding an alternate path for said magnetic flux shunting saidcompound winding, a ferrocapacitor connected across said secondarywinding, an AC voltage source connected to said primary winding, a loadcoupled to said secondary winding, an integrating circuit including anintegrating capacitor coupled to said secondary winding for developing avoltage proportional to the volt-time integral of the voltage acrosssaid ferrocapacitor, and switching means serially connected with saidcompound winding and to said integrating capacitor for completing acurrent path through said compound winding in response to apredetermined voltage across said integrating capacitor to limit theflux linking said compound winding and cause said saturating fluxshunting means to saturate, whereby the charge on said ferrocapacitor iscaused to reverse.

2. Voltage regulating apparatus as in claim 1 wherein said transfonnercore has three legs, said primary and secondary windings are wound onthe center leg of said three legs, said compound winding is wound on oneouter leg and said saturating flux shunting means comprises the otherouter leg of said three core legs.

3. Voltage regulating apparatus as in claim 2 including a saturatingwinding connected in series with said compound winding and wound uponsaid other outer leg, and a source of auxiliary voltage in phase withthe voltage across said ferrocapacitor wherein said switching meansconnects said compound winding, said saturating winding and saidauxiliary voltage source in series in response to the voltage acrosssaid integrating capacitor.

4. Voltage regulating apparatus as in claim 3 wherein said source ofauxiliary voltage is a portion of said secondary winding.

5. Voltage regulating apparatus as in claim 1 wherein said transformercore includes three legs, said primary, secondary and compound windingsare wound on the center leg of said three legs, and said saturating fluxshunting means is disposed between said secondary and said compoundwindings and extends from said center leg to said outer legs of saidthree legs.

6. Voltage regulating apparatus as in claim 5 wherein said switchingmeans short circuits said compound winding in response to apredetermined voltage across said integrating capacitor.

7. Voltage regulating apparatus as in claim 6 wherein said switchingmeans comprises an AC semiconductor switch having a conducting pathconnected in series with said compound winding and a gating pathconnected across said integrating capacitor.

8. Voltage regulating apparatus as in claim 6 including a fourth windingon said center leg closely coupled to said secondary winding whereinsaid load is connected across said fourth winding.

9. Voltage regulating apparatus as in claim 6 including feedback meansresponsive to the voltage across said load connected to said integratingcapacitor and said load to vary the charging rate of said integratingcapacitor.

10. Voltage regulating apparatus as in claim 9 wherein said integratingcircuit includes an integrating resistor, and said feedback meanscomprises an error detector connected across said load for producing anerror voltage proportional to the difference between the voltage acrosssaid load and a reference voltage, a full-wave bridge rectifier having apair of AC terminals connected across said integrating resistor and apair of DC tenninals, and unidirectional conductive means responsive tosaid error voltage connected across said DC terminals.

1 l. A ferroresonant voltage regulating circuit for regulating thevoltage at which current is delivered to a load from an alternatingcurrent source comprising a first inductance connected in series withsaid source, a second inductance coupled to said first inductance andconnected across said load, said second inductance having a corecomprising a saturating portion and a nonsaturating portion, aferrocapacitor coupled to said second inductance, an integrating circuitincluding an integrating capacitor coupled to said second inductance fordeveloping a voltage proportional to the volt-time integral of thevoltage across said ferrocapacitor, a third inductance coupled to saidsecond inductance by said nonsaturating core portion, and switchingmeans connected to said integrating capacitor and said third inductancefor completing a current path through said third inductance to causesaid saturating core portion to saturate and the charge on saidferrocapacitor to reverse in response to a predetermined voltage acrosssaid integrating capacitor.

12. A ferroresonant voltage regulating circuit as in claim 11 whereinsaid second inductance is connected in series with said first inductanceacross said source. 7

13. Voltage regulating apparatus as in claim 11 including feedback meansresponsive to the voltage across said load connected to said integratingcapacitor and said load to vary the charging rate of said integratingcapacitor.

14. Voltage regulating apparatus as in claim 1 1 wherein saidintegrating circuit includes an integrating resistor and said feedbackmeans comprises an error detector connected across said load forproducing an error voltage proportional to the difference between thevoltage across said load and a reference voltage, a full-wave bridgerectifier having a pair of AC terminals connected across saidintegrating resistor and a pair of DC terminals, and unidirectionalconductive means responsive to said error voltage connected across saidDC terminals.

1. Voltage regulating apparatus comprising a transformer having amagnetic core, primary, secondary, and compound windings wound on saidcore, said core providing a path for magnetic flux linking said primary,secondary, and compound windings, nonsaturating flux shunting meansdisposed between said primary and secondary windings to shunt a portionof said magnetic flux from said primary winding, and saturating fluxshunting means disposed between said secondary and compound windings forproviding an alternate path for said magnetic flux shunting saidcompound winding, a ferrocapacitor connected across said secondarywinding, an AC voltage source connected to said primary winding, a loadcoupled to said secondary winding, an integrating circuit including anintegrating capacitor coupled to said secondary winding for developing avoltage proportional to the volt-time integral of the voltage acrosssaid ferrocapacitor, and switching means serially connected with saidcompound winding and to said integrating capacitor for completing acurrent path through said compound winding in response to apredetermined voltage across said integrating capacitor to limit theflux linking said compound winding and cause said saturating fluxshunting means to saturate, whereby the charge on said ferrocapacitor iscaused to reverse.
 2. Voltage regulating apparatus as in claim 1 whereinsaid transformer core has three legs, said primary and secondarywindings are wound on the center leg of said three legs, said compoundwinding is wound on one outer leg and said saturating flux shuntingmeans comprises the other outer leg of said three core legs.
 3. Voltageregulating apparatus as in claim 2 including a saturating windingconnected in series with said compound winding and wound upon said otherouter leg, and a source of auxiliary voltage in phase with the voltageacross said ferrocapacitor wherein said switching means connects saidcompound winding, said saturating winding and said auxiliary voltagesource in series in response to the voltage across said integratingcapacitor.
 4. Voltage regulating apparatus as in claim 3 wherein saidsource of auxiliary voltage is a portion of said secondary winding. 5.Voltage regulating apparatus as in claim 1 wherein said transformer coreincludes three legs, said primary, secondary and compound windings arewound on the center leg of said three legs, and said saturating fluxshunting means is disposed between said secondary and said compoundwindings and extends from said center leg to said outer legs of saidthree legs.
 6. Voltage regulating apparatus as in claim 5 wherein saidswitching means short circuits said compound winding in response to apredetermined voltage across said integrating capacitor.
 7. Voltageregulating apparatus as in claim 6 wherein said switching meanscomprises an AC semiconductor switch having a conducting path connectedin series with said compound winding and a gating path connected acrosssaid integrating capacitor.
 8. Voltage regulating apparatus as in claim6 including a fourth winding on said center leg closely coupled to saidsecondary winding wherein said load is connected across said fourthwinding.
 9. Voltage regulating apparatus as in claim 6 includingfeedback means responsive to the voltage across said load connected tosaid integrating capacitor and said load to vary the charging rate ofsaid integrating capacitor.
 10. Voltage regulating apparatus as in claim9 wherein said integrating circuit includes an integrating resistor, andsaid feedback means comprises an error detector connected across saidload for producing an error voltage proportional to the differencebetween the voltage across said load and a reference voltage, afull-wave bridge rectifier having a pair of AC terminals connectedacross said integrating resistor and a pair of DC terminals, andunidirectional conductive means responsive to said error voltageconnected across said DC terminals.
 11. A ferroresonant voltageregulating circuit for regulating the voltage at which current isdelivered to a load from an alternating current source comprising afirst inductance connected in series with said source, a secondinductance coupled to said first inductance and connected across saidload, said second inductance having a core comprising a saturatingportion and a nonsaturating portion, a ferrocapacitor coupled to saidsecond inductance, an integrating circuit including an integratingcapacitor coupled to said second inductance for developing a voltageproportional to the volt-time integral of the voltage across saidferrocapacitor, a third inductance coupled to said second inductance bysaid nonsaturating core portion, and switching means connected to saidintegrating capacitor and said third inductance for completing a currentpath through said third inductance to cause said saturating core portionto saturate and the charge on said ferrocapacitor to reverse in responseto a predetermined voltage across said integrating capacitor.
 12. Aferroresonant voltage regulating circuit as in claim 11 wherein saidsecond inductance is connected in series with said first inductanceacross said source.
 13. Voltage regulating apparatus as in claim 11including feedback means responsive to the voltage across said loadconnected to said integrating capacitor and said load to vary thecharging rate of said integrating capacitor.
 14. Voltage regulatingapparatus as in claim 11 wherein said integrating circuit includes anintegrating resistor and said feedback means comprises an error detectorconnected across said load for producing an error voltage proportionalto the difference between the voltage across said load and a referencevoltage, a full-wave bridge rectifier having a pair of AC terminalsconnected across said integrating resistor and a pair of DC terminals,and unidirectional conductive means responsive to said error voltageconnected across said DC terminals.